1. Field of the Invention
The present invention relates to a method for regenerating a carrier core material for electrophotography, a method for manufacturing a carrier for electrophotography, and a carrier for electrophotography.
2. Description of the Related Art
A so-called two-component developer including a mixture of carrier particles and toner particles is frequently used in electrophotography. These carrier particles are formed, for example, from magnetic particles and a resin. These include a configuration that a coating layer including a resin as a main component is formed on a surface of relatively large magnetic particles and a configuration that a relatively small magnetic powder is uniformly dispersed in a resin.
With a conventional developer, degradation of carrier characteristics such as cracking, chipping and peeling of carrier surface and so-called spent carrier that a toner film is formed on a carrier surface due to repeated use over a long period of time has been a problem. In order to solve this problem, there have been various improvements proposed in regard to types of resins coating a carrier and crosslinking methods (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 05-127432, 05-216282, 05-216283, 05-197211, 07-114221, 08-87137 and 06-194881, and Japanese Patent Application Publication (JP-B) No. 62-61948).
In recent years, environmental destruction caused by industrial waste has become a problem, and reuse of a developer after use has become one of the challenges. However, until now, a developer after use cannot be reused and has been discarded even though carrier characteristics are improved during use.
Therefore, with respect to this reuse of a developer, a method for restoring performance by removing a spent toner on a carrier surface has been proposed. Also, a method for recovering a carrier core material by removing a resin which firmly coats a surface of a carrier core material as magnetic particles and providing again a coating resin to regenerate it as a carrier has been proposed.
For example, as the method for restoring performance by removing a spent toner on a carrier surface, a technique to remove the spent toner on the carrier surface by heating and solvent cleaning (see JP-A No. 6-149132). With this proposed technique, mainly a carrier degraded due to a spent toner may be recycled.
With this proposed technique, however, when degradation of performance is not only due to a spent toner but also due to cracking, chipping and peeling of a resin coating a surface of a carrier core material, the performance cannot be restored, or the carrier cannot be reused, only by removing the spent toner. Also, despite using this proposed technique, there is a spent toner which is difficult to remove, and thus a more powerful removing technique is desired. Further, when a carrier is washed with a solvent, a more environmentally low-impact technique is desired, considering post-processing of the solvent itself.
For example, as a method for removing a resin firmly covering a surface of a carrier core material, a technique to burn a collected developer at around 1,000° F. so as to remove the covering resin from the carrier core material has been proposed (see JP-A No. 47-12286). With this technique, it is possible to remove a coating resin from a carrier which is coated with a thermoplastic resin such as acrylic resin.
However, when a thermosetting resin is used as a coating resin of a carrier, there is a problem that decomposition of the coating resin is insufficient. Also, when a ferrite core material as a metal suboxide imparting desired magnetic properties is used and regenerated by this conventional technique, there is a problem that the original properties of the core material cannot be restored.
As described above, the conventional techniques have not achieved a method for regenerating a carrier which satisfies both conditions for removing a chemically and mechanically firm coat layer from a carrier core material and conditions not to sacrifice performance of the carrier core material imparting desired magnetic properties. Especially, since a carrier core material is usually a metal suboxide having a specific crystalline structure, chemical changes such as oxidation or changes in the crystalline structure must be avoided in a recycling process. However, for a carrier composed of particles of a metal suboxide having a specific crystalline structure and a coat layer, there has been no prior art to recover the carrier core material without involving oxidation or reduction to the oxide as well as disturbing the crystalline state, i.e. to recover the carrier core material without degrading its magnetic properties.
Meanwhile, as a technique to decompose a resin, a technique to decompose a resin in water in a supercritical state or a subcritical state is proposed (see JP-A No. 05-31000). Also, a technique to decompose a thermosetting resin in water in a supercritical state or a subcritical state is proposed (see JP-A No. 10-24274). In addition, a technique to process a chlorine-containing plastic waste using supercritical water is proposed (JP-A No. 9-111249). These proposed techniques are performed mainly for the purpose of monomerizing a large quantity of resin waste for detoxification as well as turning it into a raw material, and conditions suitable for the purpose are proposed. Thus, regeneration of a carrier core material is not considered at all.
Regarding a method for processing a carrier, a quick and efficient processing technique to treat a carrier in subcritical water at 280° C. or below containing hydrogen peroxide is proposed (see JP-A No. 2007-206614). With this proposed technique, it is disclosed that an effect of removing a coating resin is improved when a weight of a solvent with respect to a weight of the carrier is increased while maintaining a constant concentration of an oxidant.
Also, a technique to remove a coating resin without affecting magnetic properties of a carrier core material as a magnetic body by means of a method for removing a carrier coating resin using subcritical water at 280° C. or less has been proposed (see Japanese Patent (JP-B) No. 4244197).
However, in these proposed techniques, there is a problem that it takes a long time to remove a specific resin film to some extent since the decomposition condition is subcritical, which imparts low capacity of resin decomposition compared to supercritical condition. Also, it is necessary to heat for a long time, and consequently thermal energy cost increases. Further, a coating resin layer of a carrier includes an electrically conductive material such as silica, alumina and carbon black for the purpose of controlling electrical conductivity of the carrier itself. However, there is a problem with these proposed techniques that metal particles such as silica and alumina cannot be removed even though a resin may be removed to some extent. Also, with these proposed techniques, an operation to separate a toner from a developer recovered from the market is required in recycling carrier core materials, and there is a problem that the increased number of steps reduces production efficiency and increases running cost.
As described above, supercritical water or subcritical water is effective for treating an object to be processed, but it is also important to set treating conditions considering economic efficiency. High-temperature and high-pressure conditions improve decomposition performance, which is effective for treating an object to be processed, but it restricts equipment specifications, resulting in high cost.
Accordingly, it is desired at present to provide: a method for regenerating a carrier core material for electrophotography which quickly and efficiently separates and removes a resin firmly coating the carrier core material for electrophotography without excessive high-temperature and high-pressure conditions, does not affect properties of the carrier core material for electrophotography and provides sufficient performance as a carrier for electrophotography even after a resin is coated again; a carrier core material for electrophotography obtained by the method for regenerating a carrier core material for electrophotography; and a carrier for electrophotography using the carrier core material for electrophotography.